1. Field of the Invention
The present invention relates generally to an image scanning apparatus. More particularly, it is concerned with an image scanning apparatus which reads an image at a high speed using a one-dimensional image sensor having multiple arrays of photosensitive elements.
2. Related Art
Conventionally, a one-dimensional charge-coupled device (CCD) image sensor has been widely used as an image reading device of an image scanning apparatus to achieve high-speed image scanning operation, the one-dimensional CCD image sensor having a single array of photosensitive elements, multiple signal charge transfer electrodes which transfer signal charges taken from the individual elements of the photosensitive array in parallel along separate lines, and signal output sections which convert the signal charges into electric signals and output the electric signals.
The one-dimensional CCD image sensor of this kind has two signal charge transfer electrodes situated on opposite sides of the photosensitive element array, and signal charges produced by photosensitive elements designated by even numbers and those produced by photosensitive elements designated by odd numbers are transferred to the two different signal charge transfer electrodes. These signal charges are output from two signal output sections in parallel through the two-channel signal charge transfer electrodes to thereby achieve a certain degree of high-speed read-out performance. For example, an image scanning apparatus designed to read a document whose width corresponds to that of the A3 paper size (JIS: Japanese Industrial Standard) with a resolution of 400 dots per inch (dpi) employs a two-channel parallel-output one-dimensional CCD image sensor having 5000 effective pixels.
Generally, an output amplifier constituting a signal output section has limitations in frequency characteristics, and the range of signal transfer clock frequency for driving a signal charge transfer electrode is limited correspondingly. Provided that signal charges are read out with a clock frequency of about 20 MHz which is the highest clock frequency according to the frequency characteristics of currently available output amplifiers, an overall data rate achieved with a two-channel configuration would be approximately 40 MHz in terms of signal reading rate. Under these conditions, scanning speed in a slow scanning direction is theoretically 40 (MHz)÷5000 (pixels)÷15.7 (pixels/mm)≈508 (mm/s). Taking into account signal charge shift time and signal charge transfer time taken up by noneffective pixels, practical scanning speed would be approximately 450 mm/s.
With the growing needs for improving image read-out quality of recent year, an image reading resolution of 600 dpi or more has increasingly been required. A one-dimensional CCD image sensor to be used to meet this requirement should have 7500 effective pixels when scanning an A3-size document, for example. As it is necessary to increase the image reading resolution in both fast and slow scanning directions, not only the number of pixels but also the reading resolution in the slow scanning direction needs to be increased. In this situation, the signal reading rate achieved even with a two-channel parallel-output configuration is becoming insufficient in these days.
Another previous approach devised under this circumstance is to divide each of two signal charge transfer electrodes of a one-dimensional CCD image sensor into first and second half portions and individually provide output sections to the divided portions of each signal charge transfer electrode. This provides a 4-channel parallel-output configuration in which signal charges are transferred in different directions (left and right) through the divided portions of the signal charge transfer electrodes to enable even faster signal read-out operation. Provided that the signal charges are read out with a clock frequency of about 20 MHz which is the highest clock frequency according to the frequency characteristics of the currently available output amplifiers, an overall data rate achieved with this 4-channel configuration can be improved up to approximately 80 MHz in terms of signal reading rate.
If signal charges for 7500 pixels necessary for scanning a document whose width corresponds to that of the A3 size (JIS) with a resolution of 600 dpi are to be read out with the above data rate (80 MHz), a theoretical scanning speed is calculated as follows: 80 (MHz)÷7500 (pixels)÷23.6 (pixels/mm)≈452 (mm/s). Even when the signal charge shift time and signal charge transfer time taken up by noneffective pixels are taken into account, a practical scanning speed of approximately 420 mm/s would be obtained.
In the one-dimensional CCD image sensor whose signal charge transfer electrodes situated on opposite sides of a photosensitive element array are divided into the first and second half portions, which are provided with the respective output sections to transfer the signal charges to the left and right directions, forming the 4-channel parallel-output configuration as described above, it is possible to scan approximately 90 sheets of A4-size (JIS) documents per minute when the sheets are scanned in landscape, or horizontal, orientation, although there are restrictions on the scanning mechanism such as that a specific time interval is required between one document and another.
Recent years, however, have seen ever greater increases in the copying speed of digital copying machines, some achieving a printout speed of 100 sheets (A4 size) or over per minute. Nevertheless, since currently available image reading devices have lower working speeds than the printout speed for reasons stated above, it is desired to develop an image scanning apparatus capable of working at higher speeds.
Moreover, there has been a growing demand in recent years for higher resolution and higher image reading speed in image scanning apparatus other than the digital copying machines, such as in a document reader intended for reading a document by use of optical character recognition (OCR) technology.
The prior art proposes an image scanning apparatus as shown in FIG. 7 (e.g., Japanese Laid-open Patent Publication No. 9-46477) to solve the aforementioned problems, in which a halogen lamp 102 projects light onto a document placed on a platen glass 101 and light reflected by the document is guided through a mirror system 103 and a lens 104 and focused onto two one-dimensional CCD image sensors 106, 107 via a semitransparent mirror 105. With this arrangement, the image scanning apparatus simultaneously reads multiple lines of document information with the two one-dimensional CCD image sensors 106, 107 and combines the lines of document information in later signal processing to thereby realize a high-speed image scanning capability.
The aforementioned prior art technology, however, has a problem that it is difficult to achieve high-accuracy positioning of the two one-dimensional CCD image sensors 106, 107, which should be installed at separate positions. Another problem is that when the reflected light is divided by the semitransparent mirror 105, the amount of light decreases in inverse proportion to the number of divisions, making it difficult to read out signal charges with a sufficiently high signal-to-noise (S/N) ratio.